In U.S. Pat. No. 4,713,821, Bradford et al. describe a MOPA with discrete oscillator and amplifier elements. The MOPA includes a semiconductor laser diode having an output facet optically coupled to an optical power amplifier. The diode and amplifier are formed together as a single integral crystal, then cleaved to form a crack or division between the two. The diode and amplifier remain essentially a single unit in optical alignment with only a slight longitudinal shift resulting from the cleave. The active region at the entrance to the amplifier has the same lateral dimension as the active region at the exit of the laser diode, and the lateral dimension increases linearly in the forward direction toward the exit facet. The exit facet of the amplifier is antireflection coated.
In U.S. Pat. No. 4,744,089, Montroll et al. describe an improved MOPA structure which requires no cleaving into separate laser and amplifier elements, and is therefore monolithic. A laser diode has a periodic grating providing distributed feedback. A power amplifier with a diverging active area follows the laser, and the amplifier output facet is made antireflecting. The laser and amplifier are formed with separate electrical contacts so they can be driven by independent current sources.
In U.S. Pat. No. 5,003,550, Welch et al. describe a monolithic MOPA device having a steerable output beam. The device includes a single mode diode laser with distributed Bragg reflector (DBR) gratings, an optical amplifier disposed in tandem with the laser and a detuned second order grating surface output coupler disposed to receive the amplified light, all formed on a common substrate. The diode laser is tunable by means of a separate tuning current I.sub.t applied to the rear DBR grating. The amplifier is a flared waveguide coupled to receive the laser output and electrically pumped to provide optical power gain to the received light. In an alternate embodiment, the amplifier is a power splitter network of branching single mode waveguides coupled at y-junctions. The branched waveguides are followed by an array of single mode gain waveguides. The power splitter portion is pumped to compensate for scattering and splitting losses at the y-junctions, while the single mode gain waveguides are pumped to provide an optical power gain to the lightwaves. Tuning the laser, in conjunction with the surface output coupler grating, produces a longitudinal steering of the output beam. Lateral phase controllers having an array of separately addressed electrodes may be incorporated between the amplifier output and the surface output coupler to adjust the optical path length in order to enable compensation for lateral phase variations in the amplifier and to provide lateral steering of the output beam.
In U.S. Pat. No. 5,175,643, Andrews describes a monolithic MOPA device having a laser diode and preamplifier section, both with a single mode waveguide of constant width throughout, followed by a flared amplifier section increasing in width at a constant rate of flare. The laser diode, preamplifier section, and flared amplifier section are independently supplied with pumping current applied to separate contacts. The preamplifier section is modulated instead of the laser diode to avoid "chirping". The preamplifier section may also be used as a phase controller, a variable gain device for the laser or to saturate the gain at the input end of the flared amplifier section. The flared amplifier section has a single contact and so a uniform current density is applied over the entire flared amplifier.
In U.S. Pat. No. 4,965,525, Zah describes an optical amplifier having an index guided waveguide oriented so that its longitudinal axis is at a nonperpendicular angle relative to the cleaved facets. The facets are antireflection coated. The "slanted" waveguide has a relatively long straight middle portion supporting propagation of only a single spatial mode and two flared portions in the immediate vicinity of the facets. The mode width in the flared waveguide portions gradually increase from the straight portion towards the facets. In operation, the slanted orientation of the waveguide keeps internal light reflected by the facets from coupling back through the waveguide, thereby suppressing self-resonance by the amplifier. The flaring of the waveguide at its ends allows the waveguide to be formed with a greater slant angle without causing an increase in coupling losses for light input into the amplifier.
An object of the invention is to provide an optical amplifier with efficient, high power, coherence-maintaining amplifier operation together with reduced lensing or other phase distortions in broad area portions of the amplifier.
Another object of the invention is to provide an amplifier with improved modulation characteristics at high power.
A further object of the invention is to provide a MOPA device incorporating such optical amplifiers.